Method of producing metal powders of high alloy content



Sept. 17, 1946. WULFF 2,497,862

METHOD OF PRODUCING METAL POWDERS OF HIGH ALLOY CONTENT Filed March 17,1941 SENsrnzme FURNACE.

J CHARGE h.- CORROSION VAT WATER WASH a CLASSIFIER Q l DRYER MAGN ETIC QSEPHHATOR y GRADED powosa Jblzz Wui atented Sept. 17, 1946 STATES PATENTOFFICE METHOD OF PRODUCING METAL POWDERS OF HIGH ALLOY CONTENT 10Claims. 1

This invention relates to an improved method of producing metal powdersof high alloy content.

In the field of ferrous powder metallurgy there is an increasing demandfor products of improved physical and chemical characteristics, such asincreased tensile strength, corrosion resistance and the like. Suchimprovements are difficult to attain when utilizing relatively pure ironpow-- der as a starting material, for any improvement in propertieswhich is achieved is secured by expensive treatments, such as hotpressing and diffusion of secondary constituents, such as carbon and thelike, into the iron matrix.

It is well known in the field of fusion metallurgy that the mosteffective and economical method of modifying the physical andphysiochemical characteristics of ferrous base products is to combinepredetermined alloy constituents with the base metal in the melt so asto secure an ultimate alloy of the desired analysis and correspondingphysical characteristics. In the field of powder metallurgy thedesirability of utilizing alloy powders has been recognized and attemptshave been made to produce such products. The suggested methods howeverhave been so expensive as to preclude commercial production. This highexpense has been largely due to the fact that the production of thealloy has been based on a special treatment of the prefabricated ironpowder, as for example by eiTecting difiusion of one or more solid phasealloying constituents into the solid phase iron powder or powdercompact.

The present invention relates to the production of special alloy powdersby utilizing cheap, readily available scrap material containing thedesired alloy constituents and reducing this ma terial' to the powderform by utilizing what, for the sake of a term, may be called aninherent weakness in the starting material. Because of the desirablecharacteristics of stainless steel as a superior metal for theproduction of high strength, corrosion and heat resistant products thismaterial will be chosen as a medium to illustrate the fundamentalconcept of the invention. It will be understood, however, that theinvention may be availed of for the production of powder from anyalloys, whether ferrous or nonferrous, which exhibit the specialmetallurgical characteristics which are utilized as herein described.

It is known that one of the major shortcomings of austenitic steels ofthe stainless type containing approximately 18 to chromium and 8 to 20%nickel and above .02% carbon is their susceptibility to intergranularcorrosion when heated to a temperature between about 500 C. and 900 C.for an appreciable period of time. In these circumstances the carbonwhich existed in metastable solid solution is rejected in the form of achromium-rich carbide in the grain boundaries. The steel which is thussensitized is susceptible to preferential corrosion by a number ofdifferent corrodents. When subjected to such preferential boundarycorrosion the grain boundaries become weak and the erstwhile massivematerial is readily crushed to a powder.

The present invention utilizes this intergranular corrosionsusceptibility of such steels to produce an alloy steel powder ofdesirably low carbon content.

Utilizing such a concept it will be appreciated that powders may beproduced utilizing cheap equipment and with simple technique. In ordermore effectively to explain the invention, a diagrammatic illustration,in flow sheet form, is shown in the accompanying drawing.

The essential operations involved in the improved process is a heattreatment or sensitization of cheap starting material such as sheet orshot and a preferential disintegration of the boundary areas, preferablyby chemical corrodents, to produce particles corresponding to the grainsize of the material treated.

With these major steps there are employed other steps, such asmechanical disintegration, classification, brightening and/orpassivation, which improves the product for certain uses. As will beappreciated, the products produced under the principles of the inventionmay be widely utilized. The classified or graded powder may be utilizedeither alone or in a blend with iron powder to produce powder metalcompacts. be seen more fully hereinafter, chromiumnickel in alloypowders of extremely low carbon content may readily be produced by thenew method. Such powders may be compacted or briquetted with anysuitable bonding agents and may be employed as an addition agent to aniron-chromium or iron-nickel chromium melt of a relatively high carboncontent to secure an ultimate alloy of a desirably low carbon content.

The low carbon content of the products producible under the inventionrenders them exceptionally useful for welding rods. The powders may becompacted to the desired shape and coated with suitable fluxing agentsto produce welding metal of very low carbon analysis.

. The iron-chromium powders described herein As will steel scrap ispreferred. This maybe utilized in the form of borings and turnings,shotted material or sheet trim scrap. While not necessary in operatingthe process it is preferable, when sheet trim is used, to employ arelatively thin gauge scrap since the use of the thin section materialinsures a more rapid and uniform corrosion treatment. It is alsodesirable to utilize the cold rolled scrap. When hot rolled scrap isemployed the scale may be removed in a manner well known to thoseskilled in the art after the heat treatment, subsequently to bedescribed. or at any other suitable stage in the process. It is alsodesirable, although not essentially necessary, to classify the scrap tothe extent that stabilized stainless scrap, i. e. columbium and titaniumbearing stainless is culled from the charge. If such preliminaryclassification is not desirable the mixed scrap may be employed sincethe stabilized scrap is readily separable from the sensitized materialduring the course of the treatment. With certain scrap materialsintergranular corrosion may be accelerated :by heating in a carburizingatmosphere, as for example in an atmosphere of illuminating gas. I I

As is well known, the grain size of the alloy which is utilized as asource material will vary depending upon the thermal and mechanicalhistory of the scrap. A typical charge of scrap may consist only of hotrolled material or cold rolled material or a mixture of these two. Thecold rolled material may also be of diiferent degrees of reduction. Theparticle size and particle size distribution required in the ultimatepowder may,

if desired, definitely be established in the charge by making this up ofpredetermined mixtures of hot and cold rolled material which latter mayitself comprise scrap of different degrees of cold reduction. Wherecircumstances so dictate the charge may be standardized as to theeventual particle size by heat treating the mixed scrap prior to theheat sensitizing treatment; This step, as will be appreciated, may becarried out in the furnace employed for the sensitizing treatment as apreliminary step in the process. As will thus be appreciated the processis operable with run-of-mill scrap which varies widely in length andcross section as well as with smaller scrap, such as borings, turningsand the like.

The process may be carried out with simple standard equipment andchemicals and with a modicum of labor and control. As shown in thedrawing the scrap to be treated is charged to the furnace i. If desiredprior to such charging the scrap may be treated for the purpose ofcleaning it of hot scale or for any other purpose. To facilitatehandling the scrap it preferably is first pressed into a relativelyloose bale and is charged to the furnace in the form of such units.Where borings and turnings are employed it may be charged to a furnacein a suitable container.

The furnace i may be of any suitable type, such as a gas or oil firedfurnace. Typical annealing furnaces serve effectively. The onlyessential reabout 24 hours more or less.

time

quirement of the furnace is that it shall be capable of attaining andholding a temperature in the carbide precipitation range, i. e. fromabout 500 C. to- 906 C. or above.

In the furnace i the charge of scrap is heated up to the carbideprecipitation range and held within that range, preferably with cyclicvariations, for a period of time sufficient to insure completesensitizing of the stock.

The time of the sensitizing treatment, as will be appreciated, willdepend upon the characteristics of the material undergoing treatment. Ifthe charge is comprised largely of stainless of approximately .08%carbon which has an average cold reduction of 50% the heat treatment maybe continuedii or 10 hours or less. With this type of stock the furnacemay be charged and brought upto a temperature of the order of 1050 C.and held at this temperature for a period of about one half hour afterwhich the temperature may be dropped to about 800 C. and cooled slowlyfrom 880 C. to 400 C. for a period over about two hours.

In the event that the scrap is less severely cold worked the heatingcycle should be prolonged to For this type of charge the furnacepreferably is raised to a temperatureo-f between about 400 C. and 760 C.and held at this temperature for about 12 hours. The temperature is thenpreferably slowly dropped to about 400 C. within the next 12 hourperiod. In each case, as will be appreciated, the material is sensitizedin the carbide precipitation range for a period of time adequate toinsure thorough orientated carbide precipitation. Cyclic beatings arepreferred over the maintenance of a steady heat. Cyclic .heatingsbetween approximately 430 C. and 900 C. are advantageous and slowcooling from this upper temperature down to 400 C. similarly acceleratesthe desired action.

It will be understood that the heat treatment or sensitizin will also begoverned in part by the carbon content. In the preceding examples asteel scrap of about .08% carbon constituted the charge. When the carboncontent is higher the heating cycle may be shortened and when the carboncontent is lower the heating cycle should be commensurately prolonged,For very low carbon scrap, for example 115% C, the heating time must beincreased to from about two to about five times to that given in thefirst examples. When the carbon content of the scrap is low, i. e. lessthan 05% C, it is preferable to sensitize in a carburizing atmosphere.

After the scrap has been sensitized in the manner described it is thengiven a chemical disintegration treatment. If desired prior to suchchemical disintegration the material may be pick ed by any suitableagent to remove oxide. As shown in the drawing such chemicaldisintegration is accomplished by charging the treated scrap to the vat2. This vat may be of any suitable material, such as a ceramic or woodenvat lined with stabilized stainless steel or any other material which isresistant to the action of the corrodent and which does not contaminatethe powdered product. In the vat the sensitized scrap is digested in achemical solution which preferentially attacks or corrodes the grainboundaries of the sensitized metal. For this purpose a modified Strausssolution may be em ployed. This solution, for example, may comprise 47cc. H2804 (sp. g. 1.84) and 13 grams CuSO4.5H2O per liter. The solutionpreferably is held at an elevated temperature of from about 80 to 100 C.and the scrap is held in the vat for about 2 to 24 hours. In thesecircumstances the steel is well corroded along the grain boundaries. Inthe course of this corrosion particles of steel varying from fine powderup to relatively large sections detach themselves from the scrap sheetstock and fall to the bottom of the tank. When smaller material, such asborings and turnings are employed this is substantially all reduced to apowder in the vat. When sheet stock is used the quantity of thedisintegrated detached ma terial may be increased by occasionallypounding the scrap during the latter stages of the chemical corrosion.It will be appreciated that chemical solutions other than that mentionedmay be utilized for this intergranular corrosion. Thus in lieu of thesolution mentioned there may be employed such corrodents as dilutesulphuric acid, ammonium sulphate and sulphuric acid, ferric chloride,copper chloride, potassium hypochlorite, trichlorethylene, hot saltsolutions and the like. As indicated previously, any solution whichpreferentially attacks the carbide-rich grains may be utilized.

After the corrosion treatment is completed the corroded material may beremoved from the vat 2 by any suitable means and thoroughly washed. inthe container 3. The washed material from which adherent corrodent hasbeen removed may then be passed to the ball mill ll and milled thereinto reduce the larger particles to a powder. It will be appreciated thatin lieu of the ball mill shown in the drawing any other disintegratingmechanism, such as a hammer, stamp mill or tumbling mill, may beemployed. The time of treatment in the ball mill will, of course, dependupon the type of material treated and the degree to which the sensitizedproduct has been corroded. Reduction in a tumbling mill tends topreserve the powder as single crystals. Other types of milling tend tomake the powder polycrystalline. As will be appreciated, if the materialis cold worked during the distintegrating operation it tends to make theresulting powder magnetic.

After thorough disintegration in the ball mill a charge of powder isdischarged from the mill to the screen 5. The material which has notbeen powdered or pulverized is returned, as shown by line 6, to thecorrosion vat for further treatment. The material passing through thescreen 5, which preferably is less than 150 mesh, is charged to asuitable container I in which it is treated with a suitable solution,such as nitric acid which serves to dissolve the copper which plates outfrom the Strauss solution and brighten the powder as well as topassivate the stainless steel particles. For this treatment a commercialnitric acid solution may be employed. Preferably the temperature of theliquid in the vat I is maintained between about 54 C. and 60 C.

In some circumstances where the charge of scrap employed is contaminatedwith inorganic materials it may be desirable to treat the product toinsure their removal. For this purpose the material from vat 7 may bewashed and then passed to a suitable classifier 8 which may, for examplebe a Wilfley table. In this classification treatment extraneousinorganic material is removed. The classified metallic powder may thenbe discharged to the drier 9. This preferably comprises a rotary drierin which the material is dried down at temperatures maintained at 215 C.more or less.

The product which is discharged from the drier comprises metallicparticles of stainless steel intermixed with metal carbides. In order tofractionate this material and to procure a metal powder free from thecarbides the product may be treated in the magnetic separater I0. Intypical operation the carbides contained in the mass exist in the fines,i. e. the material of minus 325 mesh. When the product discharged fromthe drier is at a temperature of 215 C., which is substan' tially theCurie point of the carbides, magnetic separation is greatly facilitated.The carbide material is discharged from the separator at H and themetallic powder may be passed to the screen system I 2 from which powderof differential particle size recovered. It will be appreciated that theabove described process presents an eminently simple method of producingmetallic powder of predetermined particle size for powder metallurgyuses. Stainless steel powder so produced is singularly free from metalcarbides and oxides, these having been removed during the course of thetreatment. By reason of the freedom from oxides and carbides thematerial is extremely plastic and lends itself most readily tocompacting and sintering. In the event that the powder iscold worked toa considerable extent, as for example by reason of a long ball millingoperation, plasticity may be restored by annealing at temperatures nothigher than 900 C. for about an hour more or less and then slowlycooled. Preferably this annealing is eifected in dry hydrogen or avacuum in order to avoid undue oxidation or carburization.

Stainless steel powder has been produced in accordance with thepreceding process with eminently satisfactory results. A chargecomprised of stainless steel sheet trimmings was annealed in asensitizing furnace for a period of 24 hours during about 8 hours ofwhich the product was held within the temperature of the carbideprecipitation range. This product was then corroded for about 24 hoursin the Strauss solution previously described. Upon disintegration andclassification, i. e. the separation of the powder produced in thecorrosion treatment from the larger sections and removal of oxides andthe like an eminently satisfactory product was pro-- duced. Upon ascreen analysis this product was found to comprise 30% of powder betweenand 200 mesh, 54% between 200 and 325 mesh and 16% below 325 mesh. Thisparticle size dispersion is thus practically ideal for compacting. Acold compact made up of the powder thus produced withstood a pressure of80 tons per square inch without cracking. This, as will be appreciated,compares most favorably with typical iron powder compacts which displaya tendency to crack under a pressure of 40 to 50 tons per square inch.

In the preferred method of producing articles from the stainless steelpowder the cold pressed compact is preferably sintered in a hydrogen amosphere. When sintering at a temperature between 900" C. and 1250 C.for minutes, products have been produced which possess a tensilestrength in excess of 60,000 pounds per square inch with a reduction ofmore than 10% in area. It will be appreciated that with these improvedphysical characteristics combined with the excellent chemicalcharacteristics of stainless steel, sintered products possess a verywide permissive field of use. The stainless steel powder so producedmay, of course, be mixed or blended with other metal powder, such asiron powder, of any desire t po silver powder, nickel powder and thelike so as to produce products of varient physical, physicochemicaland/or electrical characteristics.

As will be appreciated, the physicals of the cornpacts produced underthe present invention are such as to permit subsequent processing so asto more efiectively conform the compacts to an intended use. Thesecompacts, for example may be forged and hot rolled to produce articlesof phys' ical characteristics comparable to products produced .bytypical fusion metallurgy methods.

It will be understood that the process of producing the stainless steelpowder herein described and as illustrated by the flow sheet issusceptible of many modifications. As previously explained, the processis available for use with different types of scrap, i. e. hot and coldrolled scrap. Where hot rolled scrap is used it is desirable to picklethe material at some suitable stage to remove oxides. Where oxideremoval is complete prior to passivation or drying physicalclassification or separation as illustrated at stage B may be dispensedwith. Cbviously in lieu of the particular apparatus shown othersimilarly functioning apparatus maybe employed. Again since, as pointedout, the carbides occurring in the material are largely included inthe'fines the classification in a mechanical classifier may besufficiently effective so as to preclude the necessity of magneticseparation. Again, as will be appreciated, certain of the separatestages described herein may be combined. For example, chemical corrosionand disintegration may be effective simultaneously by utilizing a ballmill or similar rotary disintegrate machine which is constructed of orlined with material which is insensitive to the corrodent, as forexample with stabilized stainless steel. In these circumstances thesensitized material may be charged to the ball mill and rotated thereinin contact with the corrodent. Again the sensitizing treatment may becarried out in a rotary kiln in which a corrosive gaseous atmosphere ismaintained.

The stainless steel powder produced as described herein may beclassified as desired so as to give any predetermined particle sizedistribution. For high pressure pressing, i. e. over 39 tons per squareinch, a powder is preferred containing about 66 of between 190 and 200mesh material, 16 more or less of between 2-00 and 325 mesh material,about 17% of between 200 and 325 mesh material and 16% below 325 mesh.For lower pressure pressing a preferred product comprises about 42% ofmaterial between 190 and 200 mesh, about 42% of material between 200 and325 mesh and about 16% of material below 325 mesh. As is understood bythose skilled in the art, in making up the powder mixture for compactlubricants, such as graphite, may be introduced. Similarly silver powdermay be utilized for its lubrication. As noted above, the particularanalysis of the powder employed for the compacting will be predeterminedupon the desired use. Thus for bearing material the stainless steelpowder may be admixed with copper or silver phosphide or low meltingsilver llOldfilS so as to improve the bearing surface. These and othermodifications and ramifications will occur to those skilled in the artin utilizing the improved product.

It is clearly to be understood, as explained earlier that the presentinvention comprehends the broad concept of producing metal powders fromcheap source material by invoking and utilizing an inherentcharacteristic of such source material for powder production. Stainlesssteel has been chosen herein as illustrative of a classical material ofthis type. The invention, however, comprehends the production of metalpowders or metal alloy powders from any source material which, by reasonof a heat treatment, is embrittled and/or rendered corrodable in theboundary areas. The invention is, therefore, applicable to any alloywhether of the solid solution type or not which through heat and/ormechanical treatment develops grain boundary weakness which thus permitsdisintegration of such boundary areas. For certain products likestainless steel separation into powder is facilitated by chemicallycorroding this differential boundary phase. In certain other alloys aliquid corrodent need not be employed and particularly where the grainboundary phase is of a brittle character. In such circumstancespowdering of the material after the heat sensitizing treatment may beeffected by any suitable type of trituration or grinding. Wheneverdesired this grinding may be in a liquid menstruum which serves as alubricant and/or as a protective material to preclude oxidation and thelike. Typical of such products which may be treated in accordance withthe invention are aluminum-magnesium alloys, aluminum silicon alloys andsimilar non-ferrous products which upon heat sensitizing or mechan icalworking develop an intergranular embrittled phase which may readily bemechanically reduced or corroded so as to produce a powder of a particlesize corresponding to the grain size of the matrix after the heattreatment. Thus magnesium aluminum alloys containing up to 10% ofaluminum may be employed. For example, brass may be heat treated andcold worked after which the product may be milled in the presence of asuitable corrodent, such as a mercurous nitrate solution of suitablestrength. Again Armco iron may be heated to about 1315" C., slowlycooled and then milled in the presence of ammonium nitrate to produce apowdered product. Other alloys amenable to the treatment to producemetallic powders are: magnesiumbismuth alloys containing up to 10%bismuth; magnesium-copper alloys containing up to 5% copper;magnesium-lead alloys containing up to 10% lead; magnesium-zinc alloyscontaining up to 8.4% zinc; and. magnesium-silicon containing up toabout 1.4% silicon. The above mentioned alloys, of course, are notexclusive or extensive but are merely illustrative of the general typecomprehended herein.

While preferred embodiments of the invention have been described it isto be understood that these are given didactically to illustrate theunderlying principles involved and not as limiting the invention to thechosen examples.

I claim:

1. A method of producing metal powder which comprises heat treating alow silicon, chromiumcontaining ferrous alloy in solid phase at atemperature sufficiently high to effect intergranular carbideprecipitation disintegrating the heat treated material and separatingthe granular from the intergranular material.

2. A method of producing metal powder which comprises heat treating achromium-containing ferrous alloy at a temperature suificiently high toeffect intergranular carbide precipitation, corroding the heat treatedmaterial and separating the granular from the intergranular material.

3. A method of producing metal powder which comprises heat treating achromium-containing ferrous alloy of the Ill-chromium 8 nickel type oflow silicon content at a temperature suificiently high to effectintergranular carbide precipitation, disintegrating the heat treatedmaterial and separating the granular from the intergranular material.

4. A method of producing stainless steel powder which comprisessensitizing stainless steel in the carbide precipitation range,selectively corroding the intergranular areas of the sensitized materialand recovering a stainless steel powder free from intergranularmaterial.

5. A method of producing stainless steel powder which comprisessensitizing stainless steel in the carbide precipitation range,selectively corroding the intergranular areas of the sensitizedmaterial, disintegrating the corroded material and recovering astainless steel powder free from intergranular material.

6. A method of producing stainless steel powder from stainless steelscrap which comprises sensitizing the material to efiect intergranularcarbide precipitation, selectively corroding the intergranular areaswith a chemical corrodent, separating the corroded mass from thecorrodent, mechanically disintegrating the corroded mass and separatingthe intergranular material from and separating the intergranularmaterial from the metal grains by gravityseparation.

8. A method of producing stainless steel pow-- der from stainless steelscrap which comprises sensitizing the material to efiect intergranularcarbide precipitation, selectively corroding the intergranular areaswith a chemical corrodent, sep-- arating the corroded mass from thecorrodent, mechanically disintegrating the corroded mass and separatingthe intergranular material from the metal grains by magnetic separation.

9. A method of producing stainless steel powder from stainless steelscrap which comprises sensitizing the material to effect intergranularcarbide precipitation, selectively corroding the intergranular areaswith a chemical corrodent, separating the solution from the corrodent,mechanically disintegrating the corroded mass and separating theintergranular material from the metal grains by sequential gravity andmagnetic separation.

10. A method of producing ferrous metal powder from standard ferrousalloys containing a component whose solubility in the alloy varies to aconsiderable extent with the temperature and which precipitates, uponheat treatment, in the form of an intergranular phase which differschemically and physically from the grains of the main component of thealloy which comprises, heat treating such alloy in solid phase at atemperature sufiiciently high to establish such intergranular phase,preferentially corroding such intergranular phase, separating thecorroded from the uncorroded material and classifying the uncorrodedmetal.

JOHN WULFF.

